Conventionally, an opening for discharging pig iron is formed by perforating wall of the blast furnance by means of a perforator. In this case, the perforator comprises a perforating rod, such as drill rod and blanking rod to thereby perform the perforating work. Since the perforating rod wears down remarkably through the perforating work, the perforating rod is necessary to be exchanged frequently and the exchanging work should be done under high temperature of the rod. A chucking device is provided in order to engage the perforating rod with an actuator or impact rod. In conventional chucking devices, the chucking device normally has a structure in which a socket is connected to the impact rod of the perforator, a perforating rod is inserted into a rod hole of the socket to be fixed thereto and the perforating rod is locked through a cotter. In this case, the exchange work of the perforating rod are usually carried out by hand.
It is preferred that the exchange work of the perforating rod is automated because of a hard work under bad conditions, such as high temperature, high dust. For the purpose, it is necessary that the cotter is movable between an engage position and disengage position and the cotter is urged against the engage position by a spring. In the conventional rod chucking device, however, vigorous vibrations are transmitted to the socket through the impact rod to vibrate the spring violently during operation As the result the spring is heated to the temperature of 300.degree. C.-400.degree. C. within a short time to produce a deterioration of material, spring constant and tensile strength resulting in losing spring function, fracture in only 20-30 minute use, and increase of wear.
Under these circumstances, it is suggested that materials having a high strength properties at high temperature, such as Inconel, are employed for the spring. However, since the spring of Inconel is very expensive and the spring is necessary to be exchanged frequently because of its short life, it will not be allowable in the cost.